Roof Pitch Angle

If you are planning to renovate your house by installing skylights or cutting new rake boards, it's essential to know the pitch of your roof. The roof pitch is mainly determined by calculating the roof pitch angle of inclination or slope of the sides of the roof surface with respect to a horizontal surface which is totally flat or parallel to the horizon. A roof is said to be pitched, if it has a gradient greater than 15 degrees or a slope greater than 3.215 in 12. The main purpose of pitched roof is to redirect the rainwater. In fact, houses in regions of low rainfall frequently have roofs of low pitch, while those in regions of high rainfall and snow, have steep roofs.

Civil engineers or carpenters use different roof pitch angle finders like a rafter to determine the pitch of a roof. One of the best way to calculate roof pitch angles is to have a range at which the roof surface will make a horizontal plane. Therefore, roof pitch and degrees like thirty degrees, forty five degrees or sixty degrees, help in constructing a building roof pitch which is in a horizontal plane.

How to Calculate Roof Pitch Angles

Roof pitch is usually expressed as a rational fraction, such as 5/12 where each number represents the coordinates of an angle. The angle is based on a roof's rise (height) and span (width). Therefore, the number 5/12 means for every 12 feet, the roof drops or rises 5 feet. You'll have to take several measurements. Therefore, here are some simple steps that will help you in calculating the roof pitch angle without taking much efforts.

• Calculate the total span by measuring the exterior of both walls. For this, hook the measuring tape to one wall and extend the tape to the outer edge of the opposite exterior wall.
• Determine the total rise by measuring from the ridge of the roof to the top plate of the house; convert this measurement in to inches to form a story pole.
• On the story pole, mark the top of the house top plate, and then put a mark higher up to determine the top of the roof ridge. The story pole can be a two-by-four board which is either marked or cut to the height of the house roof.
• Divide the total span measurement from step one by the number calculated in step two. Now convert the divided total span to feet and remaining inches to decimal feet; this will give the measurement of the total run.
• Now take the total rise from step two and divide it by total run from step four; this will give the unit rise. This measurement gives the inches by which the roof rafter will rise vertically per foot of run.
• If the rise measurement is 3 inches, the pitch is '3:12'. Now take the inverse tangent of the ratio to determine the roof pitch angle, i.e. 3:12 = atan(3/12) = 14 degrees.

Roof Pitch Angle Chart

Roof Pitch Ratio	Roof Pitch Angle
1:12	4.76°
2:12	9.46°
3:12	14.04°
4:12	18.43°
5:12	22.62°
6:12	26.57°
7:12	30.26°
8:12	33.69°
9:12	36.87°
10:12	39.81°
11:12	42.51°
12:12	45°

Therefore, by following the aforementioned steps accurately and using basic trigonometric identities, one can measure the rise and run lengths and then convert them in to respective roof slope angles.